Revert of Add Reclaim Support to ThreadLocalStorage

base/threading/thread_local_storage.cc

Index: base/threading/thread_local_storage.cc
diff --git a/base/threading/thread_local_storage.cc b/base/threading/thread_local_storage.cc
index 004d0a06cdce9dc6e68b93182c11119bbd3bad8c..0ef31f74050596e76a0647d5191e40a954965435 100644
--- a/base/threading/thread_local_storage.cc
+++ b/base/threading/thread_local_storage.cc
@@ -5,9 +5,7 @@
 #include "base/threading/thread_local_storage.h"
 
 #include "base/atomicops.h"
-#include "base/lazy_instance.h"
 #include "base/logging.h"
-#include "base/synchronization/lock.h"
 #include "build/build_config.h"
 
 using base::internal::PlatformThreadLocalStorage;
@@ -20,33 +18,37 @@
 // hold a pointer to a per-thread array (table) of slots that we allocate to
 // Chromium consumers.
 
-// g_native_tls_key is the one native TLS that we use. It stores our table.
+// g_native_tls_key is the one native TLS that we use.  It stores our table.
 base::subtle::Atomic32 g_native_tls_key =
     PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES;
 
-// The maximum number of slots in our thread local storage stack.
-constexpr int kThreadLocalStorageSize = 256;
-constexpr int kInvalidSlotValue = -1;
-
-enum TlsStatus {
-  FREE,
-  IN_USE,
-};
-
-struct TlsMetadata {
-  TlsStatus status;
-  base::ThreadLocalStorage::TLSDestructorFunc destructor;
-};
-
-// This LazyInstance isn't needed until after we've constructed the per-thread
-// TLS vector, so it's safe to use.
-base::LazyInstance<base::Lock>::Leaky g_tls_metadata_lock;
-TlsMetadata g_tls_metadata[kThreadLocalStorageSize];
-size_t g_last_assigned_slot = 0;
+// g_last_used_tls_key is the high-water-mark of allocated thread local storage.
+// Each allocation is an index into our g_tls_destructors[].  Each such index is
+// assigned to the instance variable slot_ in a ThreadLocalStorage::Slot
+// instance.  We reserve the value slot_ == 0 to indicate that the corresponding
+// instance of ThreadLocalStorage::Slot has been freed (i.e., destructor called,
+// etc.).  This reserved use of 0 is then stated as the initial value of
+// g_last_used_tls_key, so that the first issued index will be 1.
+base::subtle::Atomic32 g_last_used_tls_key = 0;
+
+// The maximum number of 'slots' in our thread local storage stack.
+const int kThreadLocalStorageSize = 256;
 
 // The maximum number of times to try to clear slots by calling destructors.
 // Use pthread naming convention for clarity.
-constexpr int kMaxDestructorIterations = kThreadLocalStorageSize;
+const int kMaxDestructorIterations = kThreadLocalStorageSize;
+
+// An array of destructor function pointers for the slots.  If a slot has a
+// destructor, it will be stored in its corresponding entry in this array.
+// The elements are volatile to ensure that when the compiler reads the value
+// to potentially call the destructor, it does so once, and that value is tested
+// for null-ness and then used. Yes, that would be a weird de-optimization,
+// but I can imagine some register machines where it was just as easy to
+// re-fetch an array element, and I want to be sure a call to free the key
+// (i.e., null out the destructor entry) that happens on a separate thread can't
+// hurt the racy calls to the destructors on another thread.
+volatile base::ThreadLocalStorage::TLSDestructorFunc
+    g_tls_destructors[kThreadLocalStorageSize];
 
 // This function is called to initialize our entire Chromium TLS system.
 // It may be called very early, and we need to complete most all of the setup
@@ -71,8 +73,8 @@
             key != PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES);
       PlatformThreadLocalStorage::FreeTLS(tmp);
     }
-    // Atomically test-and-set the tls_key. If the key is
-    // TLS_KEY_OUT_OF_INDEXES, go ahead and set it. Otherwise, do nothing, as
+    // Atomically test-and-set the tls_key.  If the key is
+    // TLS_KEY_OUT_OF_INDEXES, go ahead and set it.  Otherwise, do nothing, as
     // another thread already did our dirty work.
     if (PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES !=
         static_cast<PlatformThreadLocalStorage::TLSKey>(
@@ -88,14 +90,14 @@
   }
   CHECK(!PlatformThreadLocalStorage::GetTLSValue(key));
 
-  // Some allocators, such as TCMalloc, make use of thread local storage. As a
-  // result, any attempt to call new (or malloc) will lazily cause such a system
-  // to initialize, which will include registering for a TLS key. If we are not
-  // careful here, then that request to create a key will call new back, and
-  // we'll have an infinite loop. We avoid that as follows: Use a stack
-  // allocated vector, so that we don't have dependence on our allocator until
-  // our service is in place. (i.e., don't even call new until after we're
-  // setup)
+  // Some allocators, such as TCMalloc, make use of thread local storage.
+  // As a result, any attempt to call new (or malloc) will lazily cause such a
+  // system to initialize, which will include registering for a TLS key.  If we
+  // are not careful here, then that request to create a key will call new back,
+  // and we'll have an infinite loop.  We avoid that as follows:
+  // Use a stack allocated vector, so that we don't have dependence on our
+  // allocator until our service is in place.  (i.e., don't even call new until
+  // after we're setup)
   void* stack_allocated_tls_data[kThreadLocalStorageSize];
   memset(stack_allocated_tls_data, 0, sizeof(stack_allocated_tls_data));
   // Ensure that any rentrant calls change the temp version.
@@ -111,15 +113,15 @@
 void OnThreadExitInternal(void* value) {
   DCHECK(value);
   void** tls_data = static_cast<void**>(value);
-  // Some allocators, such as TCMalloc, use TLS. As a result, when a thread
+  // Some allocators, such as TCMalloc, use TLS.  As a result, when a thread
   // terminates, one of the destructor calls we make may be to shut down an
-  // allocator. We have to be careful that after we've shutdown all of the known
-  // destructors (perchance including an allocator), that we don't call the
-  // allocator and cause it to resurrect itself (with no possibly destructor
-  // call to follow). We handle this problem as follows: Switch to using a stack
-  // allocated vector, so that we don't have dependence on our allocator after
-  // we have called all g_tls_metadata destructors. (i.e., don't even call
-  // delete[] after we're done with destructors.)
+  // allocator.  We have to be careful that after we've shutdown all of the
+  // known destructors (perchance including an allocator), that we don't call
+  // the allocator and cause it to resurrect itself (with no possibly destructor
+  // call to follow).  We handle this problem as follows:
+  // Switch to using a stack allocated vector, so that we don't have dependence
+  // on our allocator after we have called all g_tls_destructors.  (i.e., don't
+  // even call delete[] after we're done with destructors.)
   void* stack_allocated_tls_data[kThreadLocalStorageSize];
   memcpy(stack_allocated_tls_data, tls_data, sizeof(stack_allocated_tls_data));
   // Ensure that any re-entrant calls change the temp version.
@@ -127,38 +129,33 @@
       base::subtle::NoBarrier_Load(&g_native_tls_key);
   PlatformThreadLocalStorage::SetTLSValue(key, stack_allocated_tls_data);
   delete[] tls_data;  // Our last dependence on an allocator.
-
-  // Snapshot the TLS Metadata so we don't have to lock on every access.
-  TlsMetadata tls_metadata[kThreadLocalStorageSize];
-  {
-    base::AutoLock auto_lock(g_tls_metadata_lock.Get());
-    memcpy(tls_metadata, g_tls_metadata, sizeof(g_tls_metadata));
-  }
 
   int remaining_attempts = kMaxDestructorIterations;
   bool need_to_scan_destructors = true;
   while (need_to_scan_destructors) {
     need_to_scan_destructors = false;
     // Try to destroy the first-created-slot (which is slot 1) in our last
-    // destructor call. That user was able to function, and define a slot with
+    // destructor call.  That user was able to function, and define a slot with
     // no other services running, so perhaps it is a basic service (like an
-    // allocator) and should also be destroyed last. If we get the order wrong,
-    // then we'll iterate several more times, so it is really not that critical
-    // (but it might help).
-    for (int slot = 0; slot < kThreadLocalStorageSize ; ++slot) {
+    // allocator) and should also be destroyed last.  If we get the order wrong,
+    // then we'll itterate several more times, so it is really not that
+    // critical (but it might help).
+    base::subtle::Atomic32 last_used_tls_key =
+        base::subtle::NoBarrier_Load(&g_last_used_tls_key);
+    for (int slot = last_used_tls_key; slot > 0; --slot) {
       void* tls_value = stack_allocated_tls_data[slot];
-      if (!tls_value || tls_metadata[slot].status == TlsStatus::FREE)
+      if (tls_value == NULL)
         continue;
 
       base::ThreadLocalStorage::TLSDestructorFunc destructor =
-          tls_metadata[slot].destructor;
-      if (!destructor)
+          g_tls_destructors[slot];
+      if (destructor == NULL)
         continue;
-      stack_allocated_tls_data[slot] = nullptr;  // pre-clear the slot.
+      stack_allocated_tls_data[slot] = NULL;  // pre-clear the slot.
       destructor(tls_value);
-      // Any destructor might have called a different service, which then set a
-      // different slot to a non-null value. Hence we need to check the whole
-      // vector again. This is a pthread standard.
+      // Any destructor might have called a different service, which then set
+      // a different slot to a non-NULL value.  Hence we need to check
+      // the whole vector again.  This is a pthread standard.
       need_to_scan_destructors = true;
     }
     if (--remaining_attempts <= 0) {
@@ -168,7 +165,7 @@
   }
 
   // Remove our stack allocated vector.
-  PlatformThreadLocalStorage::SetTLSValue(key, nullptr);
+  PlatformThreadLocalStorage::SetTLSValue(key, NULL);
 }
 
 }  // namespace
@@ -201,47 +198,26 @@
   PlatformThreadLocalStorage::TLSKey key =
       base::subtle::NoBarrier_Load(&g_native_tls_key);
   if (key == PlatformThreadLocalStorage::TLS_KEY_OUT_OF_INDEXES ||
-      !PlatformThreadLocalStorage::GetTLSValue(key)) {
+      !PlatformThreadLocalStorage::GetTLSValue(key))
     ConstructTlsVector();
-  }
 
   // Grab a new slot.
-  slot_ = kInvalidSlotValue;
-  {
-    base::AutoLock auto_lock(g_tls_metadata_lock.Get());
-    for (int i = 0; i < kThreadLocalStorageSize; ++i) {
-      // Tracking the last assigned slot is an attempt to find the next
-      // available slot within one iteration. Under normal usage, slots remain
-      // in use for the lifetime of the process (otherwise before we reclaimed
-      // slots, we would have run out of slots). This makes it highly likely the
-      // next slot is going to be a free slot.
-      size_t slot_candidate =
-          (g_last_assigned_slot + 1 + i) % kThreadLocalStorageSize;
-      if (g_tls_metadata[slot_candidate].status == TlsStatus::FREE) {
-        g_tls_metadata[slot_candidate].status = TlsStatus::IN_USE;
-        g_tls_metadata[slot_candidate].destructor = destructor;
-        g_last_assigned_slot = slot_candidate;
-        slot_ = slot_candidate;
-        break;
-      }
-    }
-  }
-  CHECK_NE(slot_, kInvalidSlotValue);
+  slot_ = base::subtle::NoBarrier_AtomicIncrement(&g_last_used_tls_key, 1);
+  DCHECK_GT(slot_, 0);
   CHECK_LT(slot_, kThreadLocalStorageSize);
 
   // Setup our destructor.
+  g_tls_destructors[slot_] = destructor;
   base::subtle::Release_Store(&initialized_, 1);
 }
 
 void ThreadLocalStorage::StaticSlot::Free() {
-  DCHECK_NE(slot_, kInvalidSlotValue);
+  // At this time, we don't reclaim old indices for TLS slots.
+  // So all we need to do is wipe the destructor.
+  DCHECK_GT(slot_, 0);
   DCHECK_LT(slot_, kThreadLocalStorageSize);
-  {
-    base::AutoLock auto_lock(g_tls_metadata_lock.Get());
-    g_tls_metadata[slot_].status = TlsStatus::FREE;
-    g_tls_metadata[slot_].destructor = nullptr;
-  }
-  slot_ = kInvalidSlotValue;
+  g_tls_destructors[slot_] = NULL;
+  slot_ = 0;
   base::subtle::Release_Store(&initialized_, 0);
 }
 
@@ -251,7 +227,7 @@
           base::subtle::NoBarrier_Load(&g_native_tls_key)));
   if (!tls_data)
     tls_data = ConstructTlsVector();
-  DCHECK_NE(slot_, kInvalidSlotValue);
+  DCHECK_GT(slot_, 0);
   DCHECK_LT(slot_, kThreadLocalStorageSize);
   return tls_data[slot_];
 }
@@ -262,7 +238,7 @@
           base::subtle::NoBarrier_Load(&g_native_tls_key)));
   if (!tls_data)
     tls_data = ConstructTlsVector();
-  DCHECK_NE(slot_, kInvalidSlotValue);
+  DCHECK_GT(slot_, 0);
   DCHECK_LT(slot_, kThreadLocalStorageSize);
   tls_data[slot_] = value;
 }